This invention relates to polymers having good resistance to weathering and ultra-violet degradation. This invention also relates to a method for improving the resistance to weathering and ultra-violet degradation of polymers.
Polyoxymethylene polymers, having recurring --CH.sub.2 O-- units have been known for many years. They may be prepared by the polymerization of trioxane which is a cyclic trimer of formaldehyde. Polyoxymethylene varies in molecular weight, depending on its method of preparation.
High molecular weight solid polyoxymethylenes have been prepared by polymerizing trioxane in the presence of certain fluoride catalysts such as antimony fluoride and may also be prepared in high yields and at rapid reaction rates by the use of catalysts comprising boron fluoride coordination complexes with organic compounds, as described in U.S. Pat. No. 2,989,506 of Hudgin et al. Boron fluoride gas is also a rapid and effective catalyst, as disclosed in U.S. Pat. No. 2,989,507 also of Hudgin et al. A group of oxymethylene polymers containing repeating carbon-to-carbon single bonds in the polymer chain prepared, for example, by copolymerizing trioxane and any of various cyclic ethers such as ethylene oxide or dioxalane is disclosed in U.S. Pat. No. 3,027,352 of Walling et al. Such copolymers may be described as having at least one chain containing from about 85 to about 99.6 mole percent of oxymethylene units interspersed with between about 0.4 and about 15 mole percent of --O--R-- units wherein R is a divalent radical containing at least 2 carbon atoms directly linked to each other and positioned in the chain between the two valences, with any substituents on the R radical being inert.
Other methods of preparing oxymethylene polymers are disclosed by Kern et al in Angewandte Chemi 73(6) 177-186 (Mar. 21, 1961), including polymers containing repeating carbon-to-carbon single bonds in the polymer chain by copolymerizing trioxane with cyclic ethers such as dioxane, lactones such as beta-propiolactone, anhydrides such as cyclic adipic anhydride and ethylenically unsaturated compounds such as styrene, vinyl acetate, vinyl methyl ketone, acrolein etc.
Oxymethylene polymers, when exposed to ultra-violet light for long periods of time, undergo degradation which impairs their toughness and flexibility. Further, the oxymethylene polymers become badly discolored when exposed to heat and ultra-violet light. The resulting brittleness and discoloration are undesirable, particularly in electrical and automotive applications where toughness, flexibility and color retention over prolonged periods of time are required.
It is known that the susceptibility of oxymethylene polymers to weathering without undue decrease in thermal stability can be reduced by the addition of a 2-hydroxy benzophenone to a polyoxymethylene polymer wherein the polymer units are derived from cyclic ethers having at least two adjacent carbon atoms. See in this regard U.S. Pat. No. 3,219,621 of Prichard et al, herein incorporated by reference.
It is desirable, however, to provide a polyoxymethylene molding composition which exhibits highly desirable resistance to weathering and as well as UV stability.